F-theta lens

Abstract: An f lens comprising first and second lens groups arranged in this order from the entrance pupil side, the first lens group consisting of first and second lenses cemented together, the refracting powers of the first and second lenses being one of the combinations of negative and positive, positive and positive, and positive and negative, the second lens group consisting of a third lens having a positive or negative refracting power, and the first and second lens groups satisfying the following formulae (5) to (9), wherein  i represents the power of i-th lens as numbered from the entrance pupil side standardized on the basis of the power of the whole system,  12 represents the power of the first lens group standardized on the basis of the power of the whole system, ν di represents the Abbe's number for the sodium d-line of i-th lens as numbered from the entrance pupil side, ν dL represents the Abbe's number for the sodium d-line of one of the first and second lenses having the smaller dispersive power than the other and d 12 represents the surface separation between the first and second lens groups standardized on the basis of the focal length of the whole system.

Description: This invention relates to an f lens for a scanning optical system, and more particularly to an f lens suitable for use in a scanning optical system in which a plurality of laser beams having different wavelengths are used. Description of the Prior Art Recently there has been put into wide use various instruments such as a laser beam printer in which a laser beam scanning optical system is used. In such an optical system, an f lens is used to cause a laser beam, which is deflected by a rotary polygonal mirror or the like, to scan the surface of a medium at a constant speed. The f lens is a lens which satisfies a condition that Y=f wherein  represents the incident angle of the laser beam to the lens, f represents the focal length of the lens and Y represents the height of the image formed. Conventionally the f lens is designed on the assumption that a monochromatic or single wavelength laser beam is used and accordingly the color aberration causes no problem and correction of the color aberration has been unnecessary heretofore. However, for instance, in a laser recording system in which color images are recorded on silver salt film by use of a plurality of laser beams having different wavelengths, e.g., red, green and blue laser beams, the problem of color aberration must be taken into account. As is well known in the art, the color aberration includes the axial color aberration and the lateral color aberration. The axial color aberration shifts the position of the image in the direction of the optical axis depending on the wavelength and practically does not cause a serious problem since the focal depth of the laser beam is sufficiently large. However, the lateral color aberration causes the position of the image to shift in the direction perpendicular to the optical axis depending on the wavelength, and accordingly, the beam is shifted in the scanning direction by different distance depending on the wavelength, which makes it impossible to precisely superimpose the spots of the respective colors one on another and adversely affects the quality of images. Though this problem may be overcome by operating the laser beams of the respective colors in different timings, this approach requires complicated control and is not suitable for high-speed scanning. In Japanese Patent Publication No. 61(1986)-51289, there is disclosed a technique for correcting lateral color aberration. However this technique can correct lateral color aberration only by a wavelength of 20nm and is originally developed to correct color aberration due to fluctuation in wavelength of a semiconductor laser. Accordingly it cannot be applied to a multiple wavelength scanning optical system for color image recording. An f lens disclosed in Japanese Unexamined Patent Publication No. 62(1987)-262812 can correct the lateral color aberration for two or more laser beams with a high accuracy under predetermined conditions. However the f lens comprises four elements in three lens groups. When the number of lens elements is large, the manufacturing cost increases, and at the same time, errors in processing the lens elements and assembly of the same can accumulate. Further in order to satisfy the disclosed conditions, the material for the lens elements must be selected within a narrow range. US-A-5,086,350 discloses an f lens having a first lens group consisting of first and second lenses cemented together, the refraction powers of the first and second lenses being such that the refracting power of the first lens group is negative. The f lens further comprises a second lens group comprised of a third lens having positive refracting power. The parameters of these prior art f lens are such that they fulfil the formulae (1) to (5) disclosed in the present application. This prior art f lens is used for correcting colour aberration in accordance with the fluctuation of the wavelength of a laser beam, wherein the fluctuation comprises about ± 10 nm. The features of the pre-amble of claim 1 are, in combination, known from US-A-5,086,350. SUMMARY OF THE INVENTION In view of the foregoing observations and description, the primary object of the present invention is to provide an f lens which can correct the color aberration with a high accuracy over a wide wavelength range, and consists of a small number of lens elements to permit reduction of the size of the optical system, the manufacturing cost and the error in assembly. In accordance with the present invention, an f lens is used as defined by claim 1. In accordance with the present invention, there is provided an f lens comprising first and second lens groups arranged in this order from the entrance pupil (deflecting surface) side, the first lens group consisting of first and second lenses cemented together, the refracting powers of the first and second lenses being one of the combinations of negative and positive, positive and positive, and positive and negative, the second lens group consisting of a third lens having a positive or negative refracting power, and the first and second lens groups satisfying the following formulae (1) to (5), 2≦ ν d1 -ν d2 -0.020≦Σ( i /ν di )≦0.005 35≦ν dL 1.0≦ 12 ≦2.5 0.01≦d 12 ≦0.22 wherein  i represents the power of i-th lens as numbered from the entrance pupil side standardized on the basis of the power of the whole system,  12 represents the power of the first lens group standardized on the basis of the power of the whole system, ν di represents the Abbe's number for the sodium d-line of i-th lens as numbered from the entrance pupil side, ν dL represents the Abbe's number for the sodium d-line of one of the first and second lenses having the smaller dispersive power than the other and d 12 represents the surface separation between the first and second lens groups standardized on the basis of the focal length of the whole system. The lens elements of the f lens in accordance with the present invention need not be limited to spherical lenses but may be aspheric lenses. The f lens in accordance with the present invention comprises only three lens elements in two lens groups and simple in structure. Further when the formulae (1) to (5) are satisfied, the color aberration can be corrected over a wide wavelength range. The formula (1) limits the relation between the Abbe's numbers of the first and second lenses of the first lens group, which is a cemented lens. The difference between the Abbe's numbers governs the amount of color aberration generated at the interface between the first and second lenses. When the difference between the Abbe's numbers of the first and second lenses is smaller than 2, correction of color aberration becomes difficult and at the same time, the power of the first lens group, which is a cemented lens, becomes too large (irrespective of whether it has a positive refractive power or a negative refractive power) and both the curvature of field and the f characteristic deteriorate. The formula (2) relates to correction of the axial color aberration in paraxial theory and in order to correct the axial color aberration, the value of Σ( i /ν di ) must be substantially 0. Further in order to correct the lateral color aberration, the axial color aberration must be small to a certain extent. In accordance with the second aspect of the present invention, the relation between the power of each lens and the Abbe's number thereof is limited by the formula (2) in order to correct the lateral color aberration. When the value of Σ( i /ν di ) is smaller than the lower limit, the negative lateral color aberration increases and when the value of Σ( i /ν di ) is larger than the upper limit, the positive lateral color aberration increases. The formula (3) defines the lower limit of the Abbe's number for the sodium d-line of one of the first and second lenses having the smaller dispersive power than the other. From the viewpoint of the achromatism, the more this value is, the better. When this value is smaller than the lower limit, the Abbe's number for the sodium d-line of the other lens must be smaller than that of said one lens. In such a case, said the other lens must be formed from SF glass. However the SF glass has a large refractive index, which results in a large Petzval sum and makes difficult correction of the curvature of field. The formula (4) relates to the Petzval sum and limits the amount of curvature of field. That is, when the power of the first lens group standardized on the basis of the power of the whole system is larger than the upper limit, the second lens group (the third lens) becomes a negative lens and the power of the negative lens becomes so strong that the f characteristic sharply deteriorates at the edge of the angle of view when the curvature of field and the color aberration are corrected. On the other hand, when the power of the first lens group is smaller than the lower limit, the power of the second lens group must be strong in the positive direction and accordingly the second lens group must be a meniscus lens having a small radius of curvature which is difficult to manufacture. Further it gives rise to a problem that the curvature of field becomes large and fluctuation of the lateral color aberration with change of the angle of view becomes large. Further, since the f characteristic depends on the distance from the entrance pupil as described above, the position of the first and second lens groups relative to the entrance pupil must be limited in order to correct the f characteristic. However since the position of the first lens group is more restricted by the system in which the f lens is employed and accordingly it is preferable to limit the position of the second lens system. That is, the formula (5) limits the position of the second lens group by limiting the distance between the first and second lens groups. When the distance between the first and second lens groups is smaller than the lower limit, the f characteristic becomes too large in the positive direction to correct, and when it is larger than the upper limit, the f characteristic becomes too large in the negative direction to correct and the size of the second lens group increases, which adds to the manufacturing cost of the f lens. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1A is a view showing the arrangement of the lens elements in an f lens of a first embodiment in accordance the present invention, Figure 1B is a graph showing the lateral color aberration of the f lens shown in Figure 1A for three wavelengths W1, W2 and W3, Figure 1C is a graph showing the astigmatic field curves (curvature of field) of the f lens shown in Figure 1A for the three wavelengths W1, W2 and W3, Figure 1D is a graph showing the linearity (f characteristic) of the f lens shown in Figure 1A for the three wavelengths W1, W2 and W3, Figure 2A is a view showing the arrangement of the lens elements in an f lens of a second embodiment in accordance with the present invention, Figure 2B is a graph showing the lateral color aberration of the f lens shown in Figure 2A for three wavelengths W1, W2 and W3, Figure 2C is a graph showing the astigmatic field curves (curvature of field) of the f lens shown in Figure 2A for the three wavelengths W1, W2 and W3, Figure 2D is a graph showing the linearity (f characteristic) of the f lens shown in Figure 2A for the three wavelengths W1, W2 and W3, Figure 3A is a view showing the arrangement of the lens elements in an f lens of a third embodiment in accordance with the second aspect of the present invention, Figure 3B is a graph showing the lateral color aberration of the f lens shown in Figure 3A for three wavelengths W1, W2 and W3, Figure 3C is a graph showing the astigmatic field curves (curvature of field) of the f lens shown in Figure 3A for the three wavelengths W1, W2 and W3, Figure 3D is a graph showing the linearity (f characteristic) of the f lens shown in Figure 3A for the three wavelengths W1, W2 and W3, Figure 4A is a view showing the arrangement of the lens elements in an f lens of a fourth embodiment in accordance with the second aspect of the present invention, Figure 4B is a graph showing the lateral color aberration of the f lens shown in Figure 4A for three wavelengths W1, W2 and W3, Figure 4C is a graph showing the astigmatic field curves (curvature of field) of the f lens shown in Figure 4A for the three wavelengths W1, W2 and W3, Figure 4D is a graph showing the linearity (f characteristic) of the f lens shown in Figure 4A for the three wavelengths W1, W2 and W3, Figure 5A is a view showing the arrangement of the lens elements in an f lens of a fifth embodiment in accordance with the present invention, Figure 5B is a graph showing the lateral color aberration of the f lens shown in Figure 5A for three wavelengths W1, W2 and W3, Figure 5C is a graph showing the astigmatic field curves (curvature of field) of the f lens shown in Figure 5A for the three wavelengths W1, W2 and W3, Figure 5D is a graph showing the linearity (f characteristic) of the f lens shown in Figure 5A for the three wavelengths W1, W2 and W3, Figure 6A is a view showing the arrangement of the lens elements in an f lens of a sixth embodiment in accordance with the present invention, Figure 6B is a graph showing the lateral color aberration of the f lens shown in Figure 6A for three wavelengths W1, W2 and W3, Figure 6C is a graph showing the astigmatic field curves (curvature of field) of the f lens shown in Figure 6A for the three wavelengths W1, W2 and W3, Figure 6D is a graph showing the linearity (f characteristic) of the f lens shown in Figure 6A for the three wavelengths W1, W2 and W3, Figure 7A is a view showing the arrangement of the lens elements in an f lens of a seventh embodiment in accordance with the present invention, Figure 7B is a graph showing the lateral color aberration of the f lens shown in Figure 7A for three wavelengths W1, W2 and W3, Figure 7C is a graph showing the astigmatic field curves (curvature of field) of the f lens shown in Figure 7A for the three wavelengths W1, W2 and W3, Figure 7D is a graph showing the linearity (f characteristic) of the f lens shown in Figure 7A for the three wavelengths W1, W2 and W3, Figure 8A is a view showing the arrangement of the lens elements in an f lens of an eighth embodiment in accordance with the present invention, Figure 8B is a graph showing the lateral color aberration of the f lens shown in Figure 8A for three wavelengths W1, W2 and W3, Figure 8C is a graph showing the astigmatic field curves (curvature of field) of the f lens shown in Figure 8A for the three wavelengths W1, W2 and W3, Figure 8D is a graph showing the linearity (f characteristic) of the f lens shown in Figure 8A for the three wavelengths W1, W2 and W3, Figure 9A is a view showing the arrangement of the lens elements in an f lens of a ninth embodiment in accordance with the present invention, Figure 9B is a graph showing the lateral color aberration of the f lens shown in Figure 9A for three wavelengths W1, W2 and W3, Figure 9C is a graph showing the astigmatic field curves (curvature of field) of the f lens shown in Figure 9A for the three wavelengths W1, W2 and W3, Figure 9D is a graph showing the linearity (f characteristic) of the f lens shown in Figure 9A for the three wavelengths W1, W2 and W3, Figure 10A is a view showing the arrangement of the lens elements in an f lens of a tenth embodiment in accordance with the present invention, Figure 10B is a graph showing the lateral color aberration of the f lens shown in Figure 10A for three wavelengths W1, W2 and W3, Figure 10C is a graph showing the astigmatic field curves (curvature of field) of the f lens shown in Figure 10A for the three wavelengths W1, W2 and W3, Figure 10D is a graph showing the linearity (f characteristic) of the f lens shown in Figure 10A for the three wavelengths W1, W2 and W3, Figure 11A is a view showing the arrangement of the lens elements in an f lens of an eleventh embodiment in accordance with the present invention, Figure 11B is a graph showing the lateral color aberration of the f lens shown in Figure 11A for three wavelengths W1, W2 and W3, Figure 11C is a graph showing the astigmatic field curves (curvature of field) of the f lens shown in Figure 11A for the three wavelengths W1, W2 and W3, and Figure 11D is a graph showing the linearity (f characteristic) of the f lens shown in Figure 11A for the three wavelengths W1, W2 and W3. f lenses of eleven embodiments in accordance with the present invention will be described with reference to Figures 1 (1A to 1D) to 11 (11A to 11D) and tables 1 to 11, hereinbelow. Figures 1A to 11A respectively show the arrangements of the lens elements of the f lenses of first to eleventh embodiments, and Figures 1B, 1C and 1D to 11B, 11C and 11D respectively show the lateral color aberration, the astigmatic field curves (curvature of field) and the linearity (f characteristic) of the f lenses of the first to eleventh embodiments when three laser beams having different wavelengths W1, W2 and W3 used in a scanning optical system enter the respective f lenses. The values of the wavelengths W1, W2 and W3 are shown on the top of the tables corresponding to the f lenses. Each f lens comprises first and second lens groups arranged in this order from the entrance pupil (deflecting surface) side. The first lens group consists of first and second lenses L1' and L2' cemented together, the refracting powers of the first and second lenses L1' and L2' being one of the combinations of negative and positive, positive and positive, and positive and negative. The second lens group consists of a third lens L3' having a positive or negative refracting power. The first and second lens groups satisfies the following formulae (1) to (5), 2≦ ν d1 -ν d2 -0.020≦Σ( i /ν di )≦0.005 35≦ν dL 1.0≦ 12 ≦2.5 0.01≦d 12 ≦0.22 wherein  i represents the power of i-th lens as numbered from the entrance pupil side standardized on the basis of the power of the whole system,  12 represents the power of the first lens group standardized on the basis of the power of the whole system, ν di represents the Abbe's number for the sodium d-line of i-th lens as numbered from the entrance pupil side, ν dL represents the Abbe's number for the sodium d-line of one of the first and second lenses having the smaller dispersive power than the other and d 12 represents the surface separation between the first and second lens groups standardized on the basis of the focal length of the whole system. In the first embodiment of the second aspect of the present invention shown in Figure 1A, the first lens L1' has a positive refracting power, the second lens L2' has a negative refracting power and the power of the first lens group is set close to the upper limit of the range defined by the formula (4). The radii of curvature R of the refracting surfaces, the axial surface separations d, and the refractive indexes n d and the Abbe's numbers ν d of the lenses for the sodium d-line in the f lens of this embodiment are shown in table 1. wavelength W1;680nm, W2;532nm, W3;473nm, angle of view 55.7° i Ri di n d ν d OBJ - infinity 0 infinity 0.14220 1 0.68707 0.09480 n d1 1.62280 ν d1 56.9 2 -0.12788 0.01896 n d2 1.54041 ν d2 51.0 3 -0.59246 0.03447 4 -0.16691 0.02370 n d3 1.57845 ν d3 41.7 5 -0.31521 0.85500 surface scanned infinity - |ν d1 -ν d2 |=5.9 lateral color aberration 0.067x10 -3 Σ(i/ν di )=-0.004 curvature of field 0.023 ν dL =56.9 f characteristic 1.28%  12 =2.37 d 12 =0.03447 Figures 1B to 1D respectively show the lateral color aberration, the astigmatic field curves in the scanning direction and the linearity of the f lens of the first embodiment when laser beams having wavelengths of 680nm (W1), 532nm (W2) and 473nm (W3) enter the f lens. As can be understood from Figures 1B to 1D, in the f lens of this embodiment, the lateral color aberration is well corrected for the respective wavelengths in spite of a simple structure consisting of only three lens elements in two lens groups. In the second embodiment shown in Figure 2A, the first lens L1' has a negative refracting power, the second lens L2' has a positive refracting power and the power of the first lens group is set substantially at the middle of the range defined by the formula (4). The radii of curvature R of the refracting surfaces, the axial surface separations d, and the refractive indexes n d and the Abbe's numbers ν d of the lenses for the sodium d-line in the f lens of this embodiment are shown in table 2. wavelength W1;680nm, W2;532nm, W3;473nm, angle of view 55.7° i Ri di nd ν d OBJ - infinity 0 infinity 0.14220 1 0.96791 0.01905 n d1 1.48749 ν d1 70.4 2 0.22673 0.09524 nd2 1.49700 ν d2 81.6 3 -0.36345 0.02250 4 -0.20429 0.02370 n d3 1.80450 ν d3 39.6 5 -0.27455 0.94015 surface scanned infinity - |ν d1 -ν d2 |=11.2 lateral color aberration 0.037x10 -3 Σ( i /ν di ))=-0.004 curvature of field 0.023 ν dL =81.6 f characteristic 1.35%  12 =1.86 d 12 =0.02250 Figures 2B to 2D respectively show the lateral color aberration, the astigmatic field curves in the scanning direction and the linearity of the f lens of the second embodiment when laser beams having wavelengths of 680nm (W1), 532nm (W2) and 473nm (W3) enter the f lens. As can be understood from Figures 2B to 2D, in the f lens of this embodiment, the lateral color aberration is well corrected for the respective wavelengths in spite of a simple structure consisting of only three lens elements in two lens groups. In the third embodiment shown in Figure 3A, the first lens L1' has a positive refracting power, the second lens L2' has a negative refracting power and the third lend L3' has a positive refracting power. Further the sum of the values obtained by dividing the powers of the respective lenses by the Abbe's numbers of the respective lenses is set close to the lower limit of the range defined by the formula (2) and the power of the first lens group is set close to the lower limit of the range defined by the formula (4). The radii of curvature R of the refracting surfaces, the axial surface separations d, and the refractive indexes n d and the Abbe's numbers ν d of the lenses for the sodium d-line in the f lens of this embodiment are shown in table 3. wavelength W1;680nm, W2;532nm, W3;473nm, angle of view 55.7° i Ri di nd ν d OBJ - infinity 0 infinity 0.14220 1 1.65490 0.09480 n d1 1.49700 ν d1 81.6 2 -0.12295 0.01896 n d2 1.62012 ν d2 49.8 3 -0.34948 0.05834 4 -0.14919 0.02370 n d3 1.80610 ν d3 33.3 5 -0.15976 0.97879 surface scanned infinity - |ν d1 -ν d2 | = 31.8 lateral color aberration 0.046x10 -3 Σ(i/ν di )=-0.011 curvature of field 0.023 ν dL =81.6 f characteristic 0.92%  12 =1.07 d 12 =0.05834 Figures 3B to 3D respectively show the lateral color aberration, the astigmatic field curves in the scanning direction and the linearity of the f lens of the third embodiment when laser beams having wavelengths of 680nm (W1), 532nm (W2) and 473nm (W3) enter the f lens. As can be understood from Figures 3B to 3D, in the f lens of this embodiment, the lateral color aberration is well corrected for the respective wavelengths in spite of a simple structure consisting of only three lens elements in two lens groups. In the fourth embodiment shown in Figure 4A, the first lens L1' has a positive refracting power, the second lens L2' has a negative refracting power and the surface separation d 12 between the first and second lens groups is set close to the upper limit of the range defined'by the formula (5). The radii of curvature R of the refracting surfaces, the axial surface separations d, and the refractive indexes n d and the Abbe's numbers ν d of the lenses for the sodium d-line in the f lens of this embodiment are shown in table 4. wavelength W1;680nm, W2;532nm, W3;473nm, angle of view 55.7° i Ri di nd ν d OBJ - infinity 0 infinity 0.14220 1 1.21407 0.09480 n d1 1.49700 ν d1 81.6 2 -0.11808 0.18960 n d2 1.53996 ν d2 59.7 3 -0.49715 0.21251 4 -0.16849 0.02370 n d3 1.56873 ν d3 63.1 5 -0.18138 0.75813 surface scanned infinity - |ν d1 -ν d2 | =21.9 lateral color aberration 0.018x10 -3 Σ( i /ν di )=-0.004 curvature of field 0.026 ν dL =81.6 f characteristic 1.40%  12 =1.11 d 12 =0.21251 Figures 4B to 4D respectively show the lateral color aberration, the astigmatic field curves in the scanning direction and the linearity of the f lens of the fourth embodiment when laser beams having wavelengths of 680nm (W1), 532nm (W2) and 473nm (W3) enter the f lens. As can be understood from Figures 4B to 4D, in the f lens of this embodiment, the lateral color aberration is well corrected for the respective wavelengths in spite of a simple structure consisting of only three lens elements in two lens groups. In the fifth embodiment shown in Figure 5A, both the first and second lenses L1' and L2' have positive refracting powers, the difference in the Abbe's number between the first and second lenses is set close to the upper limit of the range which is defined by the formula (1) and determined from the practical viewpoints such as manufacturing cost and the like. The radii of curvature R of the refracting surfaces, the axial surface separations d, and the refractive indexes n d and the Abbe's numbers ν d of the lenses for the sodium d-line in the f lens of this embodiment are shown in table 5. wavelength W1;680nm, W2;532nm, W3;473nm, angle of view 55.7° i Ri di nd ν d OBJ - infinity 0 infinity 0.14220 1 0.77508 0.09480 n d1 1.45650 ν d1 90.8 2 -0.46275 0.18960 n d2 1.80518 ν d2 25.5 3 -0.37110 0.02889 4 -0.21405 0.02370 n d3 1.80518 ν d3 25.5 5 -0.30344 0.92581 surface scanned infinity - |ν d1 -ν d2 | = 65.3 lateral color aberration 0.066x10 -3 Σ(i/ν di )=-0.001 curvature of field 0.026 ν dL =90.8 f characteristic 1.44%  12 =1.97 d 12 =0.02889 Figures 5B to 5D respectively show the lateral color aberration, the astigmatic field curves in the scanning direction and the linearity of the f lens of the fifth embodiment when laser beams having wavelengths of 680nm (W1), 532nm (W2) and 473nm (W3) enter the f lens. As can be understood from Figures 5B to 5D, in the f lens of this embodiment, the lateral color aberration is well corrected for the respective wavelengths in spite of a simple structure consisting of only three lens elements in two lens groups. In the sixth embodiment shown in Figure 6A, the first lens L1' has a positive refracting power and the second lens L2' has a negative refracting power. Further the sum of the values obtained by dividing the powers of the respective lenses by the Abbe's numbers of the respective lenses is set close to the upper limit of the range defined by the formula (2). The radii of curvature R of the refracting surfaces, the axial surface separations d, and the refractive. indexes n d and the Abbe's numbers ν d of the lenses for the sodium d-line in the f lens of this embodiment are shown in table 6. wavelength W1;680nm, W2;532nm, W3;473nm, angle of view 55.7° i Ri di nd ν d OBJ - infinity 0 infinity 0.14220 1 0.99859 0.09480 n d1 1.49700 ν d1 81.6 2 -0.13870 0.01896 n d2 1.50378 ν d2 66.9 3 -0.32198 0.01262 4 -0.21507 0.02370 n d3 1.60729 ν d3 49.3 5 -0.34034 0.94833 surface scanned infinity - |ν d1 -ν d2 |=14.7 lateral color aberration 0.025x10 -3 Σ(i/ν di ) =-0.001 curvature of field 0.023 ν dL =81.6 f characteristic 2.04%  12 =1.96 d 12 =0.01262 Figures 6B to 6D respectively show the lateral color aberration, the astigmatic field curves in the scanning direction and the linearity of the f lens of the sixth embodiment when laser beams having wavelengths of 680nm (W1), 532nm (W2) and 473nm (W3) enter the f lens. As can be understood from Figures 6B to 6D, in the f lens of this embodiment, the lateral color aberration is well corrected for the respective wavelengths in spite of a. simple structure consisting of only three lens elements in two lens groups. In the seventh embodiment shown in Figure 7A, the first lens L1' has a positive refracting power and the second lens L2' has a negative refracting power. Further the difference in the Abbe's number between the first and second lenses is set close to the lower limit of the range defined by the formula (1) and the Abbe's number ν dL for the sodium d-line of one of the first and second lenses having the smaller dispersive power than the other is set close to the lower limit of the range defined by the formula (3). The radii of curvature R of the refracting surfaces, the axial surface separations d, and the refractive indexes n d and the Abbe's numbers ν d of the lenses for the sodium d-line in the f lens of this embodiment are shown in table 7. wavelength W1;680nm, W2;532nm, W3;473nm, angle of view 55.7° i Ri di nd ν d OBJ - infinity 0 infinity 0.14220 1 0.63606 0.09480 n d1 1.66446 ν d1 35.9 2 -0.11059 0.01896 nd2 1.64769 ν d2 33.8 3 -0.52266 0.05358 4 -0.18894 0.02370 n d3 1.78470 ν d3 26.1 5 -0.32143 0.81368 surface scanned infinity - |ν d1 -ν d2 |=2.1 lateral color aberration 0.091x10 -3 Σ(i/ν di )=-0.008 curvature of field 0.023 ν dL =35.9 f characteristic 1.47%  12 =2.35 d 12 =0.05358 Figures 7B to 7D respectively show the lateral color aberration, the astigmatic field curves in the scanning direction and the linearity of the f lens of the seventh embodiment when laser beams having wavelengths of 680nm (W1), 532nm (W2) and 473nm (W3) enter the f lens. As can be understood from Figures 7B to 7D, in the f lens of this embodiment, the lateral color aberration is well corrected for the respective wavelengths in spite of a simple structure consisting of only three lens elements in two lens groups. As can be understood from the description above, in the f lens in accordance with the present invention, the color aberration can be corrected with a high accuracy over a wide wavelength range, and accordingly, the error in superimposing the beam spots can be reduced in the scanning optical system or the like using a plurality of laser beams having different wavelengths. Further since being simple in structure, the f lens in accordance with the present invention can be manufactured at low cost. The f lens in accordance with the present invention need not be limited to those described above but may be variously modified so long as the formulae (1) to (5) are satisfied. The f lenses of eighth to eleventh embodiments shown in Figures 8A to 8A comprise an aspheric lens represented by the formula (6). In table 8, the value designated by represents the radius of curvature on the optical axis (at the. vertex of the aspheric surface) and means that the aspheric surface has a shape defined by the following formula (6). (the same in tables 9 to 11) x=ch 2 /[1+{1+{1-(1+k)c 2 h 2 } 1/2) ]+a 1 h 4 +a 2 h 6 +a 3 h 8 +a 4 h 10 wherein x represents the length of a perpendicular line between a point on an aspheric plane at a height of h above the optical axis and a plane tangent to the aspheric plane (a plane perpendicular to the optical axis) (as standardized on the basis of the focal length of the whole system), h represents the height above the optical axis (as standardized on the basis of the focal length of the whole system), c represents the curvature at the vertex of the aspheric plane (the reciprocal of the radius of curvature =l/R), k represents a conic constant, and a 1 to a 4 respectively represents four-dimensional, six-dimensional, eight-dimensional and ten-dimensional aspheric coefficients. In the eighth embodiment shown in Figure 21A, the first surface of the first lens L1' (the entrance surface) is an aspheric surface as shown in table 21 and the first lens has a positive refracting power. The second lens L2' has a negative refracting power. Further the power of the first lens group is set close to the upper limit of the range defined by the formula (8) and the surface separation d 12 between the first and second lens groups is set substantially at the middle of the range defined by the formula (9). The radii of curvature R of the refracting surfaces, the axial surface separations d, and the refractive indexes n d and the Abbe's numbers ν d of the lenses for the sodium d-line in the f lens of this embodiment are shown in table 21. Figures 8B to 8D respectively show the lateral color aberration, the astigmatic field curves in the scanning direction and the linearity of the f lens of the eighth embodiment when laser beams having wavelengths of 680nm (W1), 532nm (W2) and 473nm (W3) enter the f lens. As can be understood from Figures 8B to 8D, in the f lens of this embodiment, the lateral color aberration is well corrected for the respective wavelengths in spite of a simple structure consisting of only three lens elements in two lens groups. In the ninth embodiment shown in Figure 9A, the second surface of the second lens L2' (the exit surface) is an aspheric surface as shown in table 9 and the second lens L2' has a positive refracting power. The first lens L1' has a negative refracting power. The radii of curvature R of the refracting surfaces, the axial surface separations d, and the refractive indexes n d and the Abbe's numbers ν d of the lenses for the sodium d-line in the f lens of this embodiment are shown in table 9. Figures 9B to 9D respectively show the lateral color aberration, the astigmatic field curves in the scanning direction and the linearity of the f lens of the ninth embodiment when laser beams having wavelengths of 680nm (W1), 532nm (W2) and 473nm (W3) enter the f lens. As can be understood from Figures 9B to 9D, in the f lens of this embodiment, the lateral color aberration is well corrected for the respective wavelengths in spite of a simple structure consisting of only three lens elements in two lens groups. In the tenth embodiment shown in Figure 23A, the first surface of the third lens L3' (the entrance surface) is an aspheric surface as shown in table 10. The power of the first lens group is set to the upper limit of the range defined by the formula (4). The radii of curvature R of the refracting surfaces, the axial surface separations d, and the refractive indexes n d and the Abbe's numbers ν d of the lenses for the sodium d-line in the f lens of this embodiment are shown in table 10. Figures 10B to 10D respectively show the lateral color aberration, the astigmatic field curves in the scanning direction and the linearity of the f lens of the tenth embodiment when laser beams having wavelengths of 680nm (W1), 532nm (W2) and 473nm (W3) enter the f lens. As can be understood from Figures 10B to 10D, in the f lens of this embodiment, the lateral color aberration is well corrected for the respective wavelengths in spite of a simple structure consisting of only three lens elements in two lens groups. In the eleventh embodiment shown in Figure 11A, the second surface (the exit surface) of the third lend L3' is an aspheric surface as shown in table 24. Further the sum of the values obtained by dividing the powers of the respective lenses by the Abbe's numbers of the respective lenses is set substantially at the middle of the range defined by the formula (2) and the power of the first lens group is set close to the lower limit of the range defined by the formula (4). The radii of curvature R of the refracting surfaces, the axial surface separations d, and the refractive indexes n d and the Abbe's numbers ν d of the lenses for the sodium d-line in the f lens of this embodiment are shown in table 11. Figures 11B to 11D respectively show the lateral color aberration, the astigmatic field curves in the scanning direction and the linearity of the f lens of the eleventh embodiment when laser beams having wavelengths of 680nm (W1), 532nm (W2) and 473nm (W3) enter the f lens. As can be understood from Figures 11B to 11D, in the f lens of this embodiment, the lateral color aberration is well corrected for the respective wavelengths in spite of a simple structure consisting of only three lens elements in two lens groups.

Claims: use of an f lens in a scanning optical system in which a pluriality of laser beams having different wavelengths are used, comprising first and second lens groups arranged in this order from the entrance pupil side, the first lens group consisting of first and second lenses cemented together, the refracting powers of the first and second lenses being one of the combinations of negative and positive, positive and positive, and positive and negative, the second lens group consisting of a third lens having a positive or negative refracting power, the first and second lens groups satisfying the following formulae (1) to (5), 2≦ ν d1 -ν d2 -0.020≦Σ( i /ν di )≦0.005 35≦ν dL 1.0≦ 12 ≦2.5 0.01≦d 12 ≦0.22 wherein  i represents the power of i-th lens as numbered from the entrance pupil side standardized on the basis of the power of the whole system, 12 represents the power of the first lens group standardized on the basis of the power of the whole system, ν di represents the Abbe's number for the sodium d-line of i-th lens as numbered from the entrance pupil side, ν dL represents the Abbe's number for the sodium d-line of one of the first and second lenses having the smaller dispersive power than the other and d 12 represents the surface separation between the first and second lens groups standardized on the basis of the focal length of the whole system i , characterised by using a plurality of laser beams having different colors.

Inventors: NOGUCHI MASARU, SHIMADA KATSUMI, TSUJITA KAZUHIRO, NOGUCHI, MASARU, SHIMADA, KATSUMI, TSUJITA, KAZUHIRO, NOGUCHI, MASARU, C/O FUJI PHOTO FILM CO. LTD., Shimada, Katsumi c/o Fuji Photo Film Co., Ltd, TSUJITA, KAZUHIRO, C/O FUJI PHOTO FILM CO. LTD.
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1994
Main classification: G02B13
IPC codes: G02B13